Currently, the performance of overlay metrology is evaluated mainly based on random error contributions such as
precision and TIS variability. With the expected shrinkage of the overlay metrology budget to < 0.5nm, it becomes
crucial to include also systematic error contributions which affect the accuracy of the metrology. Here we discuss
fundamental aspects of overlay accuracy and a methodology to improve accuracy significantly.
We identify overlay mark imperfections and their interaction with the metrology technology, as the main source of
overlay inaccuracy. The most important type of mark imperfection is mark asymmetry. Overlay mark asymmetry leads
to a geometrical ambiguity in the definition of overlay, which can be ~1nm or less. It is shown theoretically and in
simulations that the metrology may enhance the effect of overlay mark asymmetry significantly and lead to metrology
inaccuracy ~10nm, much larger than the geometrical ambiguity. The analysis is carried out for two different overlay
metrology technologies: Imaging overlay and DBO (1st order diffraction based overlay). It is demonstrated that the
sensitivity of DBO to overlay mark asymmetry is larger than the sensitivity of imaging overlay.
Finally, we show that a recently developed measurement quality metric serves as a valuable tool for improving overlay
metrology accuracy. Simulation results demonstrate that the accuracy of imaging overlay can be improved significantly
by recipe setup optimized using the quality metric. We conclude that imaging overlay metrology, complemented by
appropriate use of measurement quality metric, results in optimal overlay accuracy.